PACEMAKERS

Overview

Pacemaker manufacturers report that 26 companies have produced more than 1,500 models to date. Currently, more than 200,000 adults and children in the United States undergo new pacemaker placement each year, and nearly 2 million patients have pacemakers today. Many factors lead to confusion regarding the behavior of a device and the perioperative care of a patient with a device, especially since case reports, textbooks, and literature reviews have not kept pace with technologic developments.

A pacemaking system consists of an impulse generator and leads to carry the electrical impulse to the patient's heart. Leads are connected to the heart's chambers through the vena cava (transvenous leads) or are directly sewn onto the surface of the heart (epicardial leads). Leads can be unipolar (one electrode per lead), bipolar (two electrodes per lead), or multipolar (multiple electrodes and wires contained within one lead with connections in multiple chambers) ( Fig. 35-1 ). Because two electrodes are required to complete a circuit, the second electrode in a unipolar configuration will be the metal generator case. Use of the case as an electrode requires that the generator pocket be devoid of gas, and electrical continuity has reportedly been disrupted by the use of nitrous oxide. ^[7]

Pacemakers with unipolar leads seem to be more sensitive to the effects of electromagnetic interference (EMI), and these systems produce larger "spikes" on an analogue-recorded electrocardiogram. Most pacemaking systems (except Autocapture devices from St. Jude Cardiac Rhythm Management, Sylmar, CA) use a bipolar pacing configuration, since bipolar pacing usually requires less energy. Also, bipolar sensing is more resistant to interference from muscle artifacts or stray electromagnetic fields. Often, bipolar electrodes can be identified on the chest film since they will have a ring electrode 1 to 3 cm proximal to the lead tip (see Fig. 35-1 ). However, generators with bipolar leads can be programmed to the unipolar mode for pacing, sensing, or both.